RE: Filesystem for Linux production database server?

> As far as the max-readahead option, I don't understand the specific> relevance to fibre-attached storage. I would imagine, though admittedly> I no longer have large storage arrays to play with, that setting> readahead to be higher could damage performance against arrays with> "intelligent" caching algorithms. Having the OS handle read-ahead

this has less to do with the caching than bw utilization. (someone ought to
test for async) and/or for filesystem io. Also, this is only for reads
(obviously).

nutshell is that the default fs *max* readahead is the current block (4k) +
31blocks for a total of ~128k. If you mess with a lot of io to fibre arrays
(through fs or raw) you'll find that 128k sized io's is right at the base of
the ramp-up for efficiently using your bus bandwidth for streaming io's (e.g.
range or fast full scans).

If you look at what happens, you start with current block + min-readahead
blocks (default is 7) for a total of 64k. if the readahead is successful, then
it doubles the number of pages read ahead for that file. repeat until you are
at max-readahead.

This PITA is that you really are trading streaming io throughput for a
percentage of random io with a caveat that a higher readahead change much if
you don't do that much sequential io happening (e.g. an OLTP database).

> rather than the array will likely fool the array into thinking that the> i/o patterns are more sequential than they are. This will cause them to> pre-allocate cache regions and pre-fetch more tracks off disk, which> could adversely impact performance.

again, this comes back to *know* your io pattern. A large db_multiblock_read
setting can yield similar results if used incorrectly (the trick with fs
readahead is that there is variance)

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Author: Craig I. Hagan
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